The quality of an image presented on a digital image display device is determined by the chromatic resolution and the spatial resolution of the display device. The chromatic resolution is defined by the number of bits needed to memorize an element of the image (pixel) and the spatial resolution is defined by the number of pixels of the display device. A typical display system, known as “true color”, makes it possible to associate with each point of the display device a digital value that directly defines the color to be displayed. For example, the color of a pixel may be represented by a digital value of 24 bits, that is to say, the intensity of each primary color (R, G, B) is represented by 8 bits.
Certain applications require one to convert a digital image of high chromatic resolution into a digital image of a lower resolution, for example, when one wants to utilize a small-size display device and/or wants to utilize a video memory of reduced dimensions for storing the data that define the image. Various known techniques make it possible to obtain quality images even in these cases, notwithstanding the low chromatic resolution actually available, by exploiting the capacity of the human eye to merge the tonal or chromatic values of adjacent pixels to perceive an intermediate tonal value or color. One of these techniques, known as “ordered dithering”, makes it possible to represent, for each primary color level of the image, the entire range of intensity values by means of image elements having only one of two possible intensity levels. The choice of one or the other intensity level is made by examining the image with a bidimensional matrix of predetermined threshold values [dither matrix]: the value of each image element is compared with the corresponding threshold value of the dither matrix to assume one of the two predetermined values on the basis of the outcome of the comparison. The dither matrix is typically a square matrix of size 4×4 or 8×8 that is repeatedly applied to the image to be processed and in such a way as to cover it completely. The same operation is repeated for all the color levels that make up the image.
Various methods are known for putting the techniques of ordered dithering into practice, but all call for the use of complex hardware and/or software systems.
There is thus a strongly felt need for methods that are less demanding as far as hardware and/or software resources are concerned.